This invention relates to an arc welder and especially, to an improved arc welder which can be used when its input power supply voltage is not only a high voltage but also a low voltage which is about a half of the high voltage.
Such a power supply device for arc welder which can be operated with either of two kinds of input power supply voltage is disclosed, for example, in the Japanese opened patent gazette No. S56-80373. This device includes a power transformer having a center-tapped primary winding, a voltage detector for detecting the input voltage and a controlled switch responsive to a detection signal of the detector for switching between the center and end taps of the power transformer and is arranged to generate a nearly same voltage across the secondary winding of the transformer regardless of the input voltage and rectify this secondary voltage to obtain a low voltage d.c. output for welding. Although this device can automatically conform to two kinds of input power supply and also be applied not only to single-phase a.c. power but also to three-phase a.c. power, it can not be expected to make the device small in both size and weight since the power transformer is large in both size and weight.
An arc welder proposed in the Japanese opened patent gazette No H1-151975 for removing this problem comprises, instead of the power transformer, an input rectifier of diode bridge type for rectifying an a.c. input, a pair of smoothing capacitors for smoothing the rectified output, an invertor having a half-bridge configuration of switching transistors for converting the d.c output into a high frequency a.c. power, an output transformer for transforming the output of the invertor into a suitable voltage and a rectifier for rectifying the output of the transformer, and the input rectifier has two pairs of input terminals for high and low voltages, respectively, and is arranged to subject its a.c. input to full-wave rectification at the time of high voltage and to voltage doubling rectifcation at the time of low voltage, thereby constantly keeping always the input of the invertor. This device can be made small in size and weight since the heavy power transformer has been substituted with a light output transformer. However, the power supply must be manually connected to either of the two pairs of input terminals in accordance with its voltage and, moreover, it cannot be applied to a three-phase a.c. power supply.
Accordingly, an object of this invention is to provide a small-sized and light-weighted arc welder which can automatically conform to two kinds of a.c. power supply voltage and use an a.c. power supply not only of single phase, but also of three phases.